Using active devices in their nonlinear region improves the operating efficiency of wireless products. Traditionally, characterizing nonlinear device behavior has involved the use of measurements and modeling to achieve optimum results. For measurements, large-signal network analyzers (LSNAs) and nonlinear vector network analyzers (NVNAs) have attempted to address this market with application software and systems to assist with the behavioral measurements.

Tektronix, Cardiff University, and Mesuro Limited are developing active harmonic load and source pull measurement systems for nonlinear device characterization. The following questions and answers should help explain the technology, which may be unfamiliar to many designers.

How does Tektronix approach nonlinear device behavior characterization?

Nonlinear device characterization measurements are no longer limited to NVNAs and LSNAs. Advances in broadband signal generators and analyzers enable new levels of nonlinear device measurements in key targeted applications. This new approach reduces system complexity for nonlinear device characterization, while its modular concept offers a high degree of flexibility to adapt to future market developments, protecting customers’ investment.

What specific instruments are involved?

Sampling scopes enable phase-coherent wideband measurements with enough dynamic range to fully characterize device behavior. The advantages of using the sampling scope include the coherent alignment of all spectral components of multiple signals that are measured simultaneously. The sampling scope can acquire up to eight signals simultaneously, making it readily expandable to measure devices with up to four single-ended or two differential ports and frequencies from dc to more than 70 GHz.

In addition, the acquisition units measure all relevant spectral components within the signal. These include the fundamental and multiple higher-order harmonics, as well as the dc and baseband response [2, 3], which are essential in capturing the often seen memory effects in devices. As a result, genuine voltage and current waveforms that represent the actual physical properties of the device are obtained.

What signal sources are used?

Arbitrary waveform generators (AWG) enable the injection of any spectral components into the input and output of the device under test and manipulation of the current and voltage waveform that exists at the device. In other words, they enable a complete source and load-pull solution not only at the fundamental and harmonic frequencies but also at baseband spectral components. This source and load-pull system, completely based on electronic instruments, adds speed and flexibility to the acquisition of load-pull data compared to mechanical load-pull systems.

Tektronix’s AWG7000 series AWGs can create and fully control wideband phase coherent multi-channel signals for the baseband, fundamental frequency, and several harmonic frequencies. All signals are sample-aligned and phase-coherent so there is no need for synchronizing multiple signal generators, extensive phase calibration, or multiple harmonic tuners.

What is the measurement process?

The integrated system allows the measurement of nonlinear parameters via genuine current and voltage waveforms to obtain an accurate picture of the device under test (DUT) behavior. The measured results enable insight into the investigation and development of efficient power-amplifier (PA) modes of operation [1] and for advanced characterization of memory effects [2, 3].

The measured results can be readily integrated into nonlinear EDA software [4], resulting in novel simulations that combine simulations and measurements [5]. The user can now determine whether a given device is better represented within the simulator through a set of nonlinear measurements or a nonlinear model.

What is Mesuro’s part of the process?

The Mesuro MB Series Active Harmonic Load Pull Systems are dedicated measurement systems that enable complete nonlinear characterization and design. It offers a new alternative to traditional VNA-based measurement techniques that measure only a single frequency component at a time. It fully recognizes that nonlinear devices and systems produce spectrally rich signals at baseband, fundamental, and harmonic frequencies and enables their simultaneous control to obtain maximum performance from a given technology.

The MB Series solution’s modular approach fully accounts for the diversity of the market spanning small, medium, and large power applications offering solutions for markets operating up to 20 and 150 W. The technology is not limited to tonal stimulus/response techniques, but can be adapted for modulated or pulsed stimulus/response measurements with software.

The capabilities of the Mesuro solution make it very relevant to the semiconductor industry as specific waveforms can be generated to test and investigate particular properties of a transistor, such as its knee-walk-out or voltage breakdown characteristics [6]. In essence, the Mesuro measurement solution is a practical realization of a harmonic balance or envelope simulator and offers the capability for a seamless integration with any nonlinear EDA software (see the figure).

Just what is load pull anyway?

Load-pull solutions have been around for a little over 10 years. Load (or source) pull is the process of altering, or pulling, the impedances at the output (or input) of an RF device while measuring the device behavior. For harmonic load pull, as a device (PA transistor) under load distorts in a nonlinear fashion, harmonic products are created with varying impedances at harmonics of the fundamental frequency being stimulated. By controlling the impedances a DUT sees as it is driven into nonlinear regions, a resulting measurement system can help predict the behavior and pre-emphasis required to optimize linear performance.

What are the advantages of active harmonic load pull, and how can this technique close the gap between predicted and measured results?

The realization of a highly efficient PA is fundamentally coupled to the precise control of the fundamental and harmonic impedances that are presented to the device. Traditional measurement systems enable systematic load-pull sweeps at the fundamental frequency with typical measurements, including device measurements at several hundred impedance points.

Extending the same approach to harmonic load-pull measurements would require the measurement of all the impedance points of the fundamental and harmonic impedances at all possible combinations. This approach is rather prohibitive as a simple grid of 100 impedances at the fundamental, second, and third harmonic would lead to 1003 = 1 million measurements. Adding other essential sweep parameters such as input power, frequency, and bias would further extend the measurement time by orders of magnitude.

Mesuro’s measurement solution cuts down the characterization time through the simultaneous utilization of its waveform measurement and harmonic load-pull capability, a functionality Mesuro calls “waveform engineering.” The design approach is based on the fact that theoretical current and voltage waveforms for all amplifier modes of operation are well defined.

Considering this allows for directed changes to the fundamental and harmonic load impedances to be made to continuously reduce the difference between the measured and theoretical waveform. This approach eliminates the need for systematic yet time-consuming multi-dimensional parameter sweeps and gives the designer a more directed approach with the added benefit that the resulting device performance is close to a theoretical optimum.

Can you provide some examples illustrating this technique?

Waveform measurement and waveform engineering can be directly applied to the analysis of different types of bias and operation techniques. For example, an inverse Class F amplifier employs a termination technique in which the even harmonics appear as an open circuit and odd harmonics as a short circuit. With a theoretical infinite series of harmonic terminations, the drain voltage will approach a half-sinusoid waveform, and the current waveform will approach a square wave.

This new design approach has been applied to two inverse Class F PA designs centered at 0.9 and 2.15 GHz. The chosen transistor was a commercially available 10-W gallium-nitride (GaN) device from Cree Inc. Optimum input and output bias conditions, fundamental impedance, and the second and third harmonic impedances were identified in the first step for an inverse Class F mode of operation utilizing the Mesuro system.